This invention has relation to a pair of hydraulic grapple arms mounted to the hydraulically operated boom of a logging or skidder vehicle. The vehicle is backed up to a plurality of logs or tree stems to be skidded, the grapple arms of a grapple assembly are opened, and the boom is lowered to bring the grapple arms down around the outside of the logs, the grapple arms are closed to encompass the logs, the boom is elevated to elevate the grapple assembly and the vehicle skids the logs to a location for use.
In order that the grapple arms can encompass the logs with ease, regardless of whether or not the logs are lying in longitudinal alignment with the skidder vehicle, it is advantageous to have control over the angular position of the plane of the grapple arms so that they can be positioned substantially perpendicular to the longitudinal alignment of the majority of the logs to be encompassed.
In one instance, as disclosed in the patent to Barwise, U.S. Pat. No. 3,841,507, granted in October of 1974, before the present invention, a flexible connector was utilized between the point of the boom and a grapple block from which the grapple arms are pivotally mounted. In order for these grapple arms to be placed to encompass the end portions of logs or tree stems, the grapple arms had to be opened and then cast in an outward direction much the same as a bucket on a dragline has to be cast only on a smaller scale. In some instances, unless the operator exercised a substantial degree of skill, the grapple assembly would turn off or twist about a vertical axis, thereby often necessitating two or three different casts before placing the grapple arms on either side of the load. Also, such grapple arms are easily deflected by limbs, brush or the like. While many operators soon developed the skills which assured efficient use of this early form of the Barwise grapple, skidder grapple owners expressed a desire for a mechanical control over the plane of the grapple arms to reduce reliance on operator experience, training and skill.
A further difficulty with the free swinging connector between the boom point and the grapple block occurred during travel of the skidder vehicle over rough terrain on the "empty" half of the log skidding cycle, from the point where the grapple unloaded the logs to the point where the next load of logs was to be picked up. During this half of the cycle, rough terrain causes excessive movement and banging of the empty grapple assembly about all axes causing undue wear, stress and shock loading on the grapple assembly, on the connector between that assembly and the mast, and on the mast itself.
Also, a simple positive rotating mechanism to control the plane of the grapple arms is needed on certain skidders, due to their particular design. Such a control mechanism is needed on all skidders on particularly rough or steep terrain, in order to maximize the efficiency of the grapple skidder combination.
Others have attempted to develop structures to positively change and control the plane of grapple arms depending from boom points on skidder vehicles. These attempts included the use of a hydraulic linear motor driving a rack to rotate a pinion. Also proposed has been the use of two linear motors, three sprockets and a sprocket chain to swing the grapple assembly. Both such types of structures are much larger, more cumbersome and more complicated than the structure of the present invention.
One prior art form of rotating mechanism for a grapple assembly utilizes a high torque, low RPM rotating hydraulic motor with a sprocket and chain speed reduction arrangement to rotate a grapple connector link connected to a grapple assembly by a single universal joint, this shaft being supported in a housing at the point of a vehicle-mounted boom on a tapered thrust bearing. With such a structure, as the vehicle turns when carrying a long load, any movement of the grapple about a vertical axis is counteracted by hydraulic pressure in the system and this pressure must be exhausted over the relief valve in the system. The hydraulic pressure, due to the speed reduction through the chain-sprocket arrangement, must be relatively high in order to develop the necessary torque on the shaft to rotate the grapple. This operating at high pressure shortens the life of the hydraulic system and can cause accelerated wear on the tapered bearing.
Also, this causes excessive heating of the hydraulic fluid which represents wasted energy.
In order to provide a simplified, long-lasting, energy efficient, inexpensive structure for rotating grapple arms with respect to the point of the boom from which they are suspended, the structure of the present invention was developed.
Applicant and those in privity with him know of no closer prior art than that discussed above; and they know of no prior art which anticipates the claims made in this application. No formal search of the prior art has been made on behalf of the applicant.